The present invention relates to a global positioning system (GPS) and receiver simulation, a gyro and accelerometer (IMU (inertial measurement unit)) simulation, and more particularly to a coupled real-time GPS/IMU simulation method with Differential GPS for a hardware-in-the-loop integrated positioning system test on the ground.
Due to budget limitations and very large time consumption for testing results, problems with the ground tests and laboratory hardware-in-the-loop tests for the on-board coupled global positioning system/inertial navigation system (GPS/INS) remain unsolved.
For ground tests, the inertial sensor within the on-board coupled GPS/IMU (Global Positioning System/Inertial Measurement Unit) can not produce dynamic electronic signals without vehicle maneuvering. This is based on the knowledge that the IMU is a self-contained device and in the static case, the global positioning system receiver (GPSR) can not output dynamic measurements. Thus, for static tests the accuracy of the on-board coupled GPS/INS for a vehicle can not be evaluated. Therefore, in order to apply the coupled GPS/INS to the vehicles, a series of dynamic tests for the fully coupled positioning system (GPS/INS) have to be executed before the xe2x80x9crealxe2x80x9d missions are activated.
As regards the expense of a test, a dynamic test (measuring the dynamic data) on the ground for the on-board fully coupled positioning system can be performed at a low budget. However, for an actual-fly test, not only is the operational cost high, but also is time/labor-consuming. Because it is necessary that the essential parts of the gyros, accelerometers, and GPS receiver experience a trajectory identical to that expected by the real mission, simulation tests are the key to meet the standards of reliable testing performed with in a low budget and with time savings.
A straightforward method for generating dynamic inertial measurements is to put an actual inertial sensor on a motion table. This method is reliable but has several disadvantages. They are described as follows:
1. A large set of testing equipment.
2. Expensive operational costs.
3. Limited dynamic-motion.
4. Inconvenient data acquisition process.
5. Inability to perform simultaneous generation of dynamic GPS measurements.
Some systems for GPS signal simulation generate suppressed radio frequency (RF) analog signals to test a GPS receiver. The RF output mimics the GPS signal emitted from the GPS satellites with modulating pseudo random noise code and navigation message data (such as ephemeris, clock parameters, and even atmospheric data) on an L-band carrier (1,575.42 MHz) or two L-band carriers (1,575.42 MHz and 1,227.60 MHz). The simulated signal has the same amplitude and signal-to-noise ratio (SNR) as the GPS signal so that it can be directly injected into the GPS receiver through the antenna port.
In order to simulate the real GPS measurements with high fidelity, a 6DOF trajectory generator which provides real-time trajectory data, real-time GPS satellite constellation simulation, selective availability (SA) simulation, intermediate frequency (IF) signal generation, and GPS receiver tracking loop simulation are used to generate simulated GPS measurement data (pseudorange, phase, and Doppler shift) based on the GPS model and receiver model. Also, the output of the simulated GPS measurement data is formatted. At the same time, using the 6DOF trajectory data, dynamic gyro and accelerometer measurements are generated according to their corresponding measurement and error models. Then, the simulated measurement data are input to the on-board fully coupled positioning system. Therefore, using the dynamic hardware-in-the-loop test, the fully coupled positioning system can be evaluated in the laboratory during the whole procedure (i.e. from measuring the data to validating the fully coupled positioning system).
The present invention is an extension of the U.S. Patent Application entitled xe2x80x9cCoupled Real-time Emulation Method for Positioning and Location Systemxe2x80x9d now allowed, wherein the simulation method is based on one GPS receiver. Using the pseudorange measurements, a Kalman filter is used to estimate a stand-alone receiver position and, therefore, the accuracy of the stand-alone estimated receiver position is usually in the level of 100 meters. Therefore, the performance of the previous patent can only be used to evaluate the low accurate fully coupled positioning systems.
An objective of the present invention is that, in addition to using the Kalman filter to estimate a stand-alone receiver position, the differential GPS simulation (based on two receivers) is employed as one of the features, so as to not only increase the domain in applications (either one receiver or two receivers) but also provide the evaluation of the fully coupled positioning system for high accuracy positioning.
Another objective of the present invention is to apply the differential GPS positioning to the fully coupled positioning system in order to test the reliability of the system for high accuracy positioning. Using the simulated reference and rover data and giving the position of the reference site, the rover position is estimated in the differential filter which comprises a plurality of Kalman filters running in parallel. The input data of the differential filter are the simulated phase measurements (reference and rover).
Another objective of the present invention is to provide reliable real-time simulated dynamic GPS and inertial (accelerometer and gyro) measurements and then to evaluate the fully coupled positioning system with a low budget and time/labor savings. Using simulated GPS and inertial measurements, the evaluation of the on-board fully coupled positioning system can be performed in the laboratory without a field experiment. Therefore, the cost of the field experiment can be saved especially for an actual-fly test, and at the same time time/labor of preparation for the field experiment can be saved.
Another objective of the present invention is that in addition to simulated rover positions which are the output of the 6DOF trajectory generator, the reference position is also simulated based on the 6DOF trajectory data. Thus, based on the real-time GPS satellite constellation simulation, IF signal generation, SA simulation, GPS receiver tracking loop simulation, receiver (reference or rover) position, and the GPS and receiver models, the corresponding real-time measurement data (pseudoranges, phases, and Doppler shifts) are generated synchronously for both reference and rover sites.
Another objective of the present invention is that based on the gyro and accelerometer measurement and error models and the 6DOF trajectory data, the IMU simulation method generates the dynamic inertial measurements without field experiments synchronously with the GPS simulated measurements.
Another objective of the present invention is that the 6DOF trajectory data can be replaced with real mission trajectories. As a result, the fully coupled positioning system can be evaluated using different cases (trajectories). In other words, the fully coupled positioning system can experience a serial of dynamic tests without limit.
Another objective of the present invention is to support the development, debugging, and final integration of the fully coupled positioning system. Therefore, the final fully coupled positioning system will work properly in applications. Thus, the present invention focuses its attention on a method to validate the gyro, accelerometer, and global positioning system coupled simulation.
Another objective of the present invention is to comprise at least one computer used as computing platforms for the coupled positioning system simulation. For the one-computer case, the computer simulates the GPS and inertial (gyro and accelerometer) in the serial mode. For the two-computers case, one is for the GPS simulation and the other is for the gyro and accelerometer simulation execution simultaneously. Moreover, several RS-232 serial ports are used to output the simulated GPS and inertial measurement data.